The present invention relates to wireless telecommunication networks. More particularly, and not by way of limitation, the present invention is directed to a method and apparatus for optimal selection of redundancy versions (RVs) for hybrid automatic repeat request (HARQ) operations in a Universal Mobile Telecommunications System (UMTS) High Speed Downlink Packet Access (HSDPA) transmission.
HARQ combines forward error correction (FEC) and automatic repeat request (ARQ) to achieve high data throughput. To place the present invention in context, a brief description of ARQ, FEC and HARQ is given below. ARQ is an error control scheme that relies on retransmitting data that is received with errors. In ARQ systems, messages are divided into blocks that are transmitted after a small number of parity bits or redundant bits have been added. The receiver uses the parity bits to detect errors that may have occurred during transmission. If errors are detected, the receiver requests a retransmission of the data blocks containing errors. ARQ is simple and achieves reasonable throughput when the error rate is low. Throughput diminishes, however, as the error rate increases because of the need to resend more data. FEC employs error-correcting codes to detect and correct errors that occur during transmission by adding redundancy to the information bits before they are transmitted. Shannon's channel coding theorem states that there always exists a coding scheme that enables information to be transmitted with arbitrarily small error probabilities provided that the data rate (including that due to the added redundancy) over the communication channel is less than the channel capacity. The redundancy enables the receiver to detect and correct errors without having to retransmit the information bits. FEC achieves a constant throughput rate regardless of error rate. However, because of fading channel condition and possible inaccuracy in link adaptation, the probability of a decoding error in systems employing FEC only can be greater than the probability of an undetected error in ARQ systems. To obtain high system reliability, a long powerful code that increases system complexity and expense may be required. HARQ systems combine ARQ and FEC to improve throughput as compared to pure ARQ systems with less complexity than pure FEC systems. The basic idea underlying HARQ is to use FEC to first detect and correct errors, and, if the errors are not correctable, to request retransmission. HARQ systems use an error correction code as an inner code and an error detection code as an outer code. If the number of errors in the message is within the capabilities of the error correction code, the errors will be corrected without the need for retransmission. If, however, the number of errors in the message exceeds the capabilities of the error correcting code, the receiver requests retransmission of the message.
Two types of HARQ modes are conventionally used. When higher order modulations (HOM), such as, but not limited to 16-ary Quadrature Amplitude Modulation (16QAM), are used in HARQ, a variation to the type-I HARQ is also enabled.
In a type-I HARQ system, a coded packet is transmitted initially and, if the receiver fails to decode the packet, a retransmission request in the form of non-acknowledgment (NACK) is fed back to the transmitter. Upon reception of this NACK, the transmitter sends the same coded packet again. This type of HARQ is commonly referred to as Chase combining (CC) in the wireless industry.
In the type-II HARQ scheme, instead of sending the same coded packets, the transmitter attempts to construct and send additional coded parities when a NACK is received. This is also known as an incremental redundancy (IR) scheme.
When HOMs are used, a third variation to the type-I HARQ is enabled by transmitting the same coded bits but in conjunction with a different bit-to-symbol mapping. For instance, four exemplary choices of such mappings 101, 102, 103, 104 for 16QAM are illustrated in FIG. 1. This is referred to as the bit-remapped Chase combining (BRMCC).
Based on a simplified assumption of the exact operational details, the following factors that affect gains and relative advantages of HARQ protocols have been identified:
r1: the initial coding rate of the packet or block to be transmitted. The higher the initial coding rate, the higher the IR gains. For higher order modulations, the BRMCC gains also increase with the initial coding rate in general. In general, IR is preferred with high r1 and BRMCC is preferred for HOMs with low r1;
r0: the mother code rate from which HARQ operation is derived. The higher the mother code rate, the lower the IR gains; and
SNR: the signal-to-noise ratio. The faster the SNR changes between transmissions, the lower the gains of IR and BRMCC. It has been shown that systematic bits of the turbo codes should receive higher protection but not highest priority.
Guidelines for type-II HARQ adaptation based on ideal behaviors of the rate matching (RM) agent that constructs different RVs are generally known. In particular, it is assumed that such a RM agent shall provide as many not-yet-used coded bits when instructed to operate in the IR mode. For instance, in UMTS, the mother code rate is normally r0=⅓. Hence, if an initial transmission with code rate r1=0.8 is reported as NACK, an ideal RM agent for IR operation shall be able to provide a RV consisting of unused coded bits only.
However, the exact behaviors of the HSDPA RM agent as defined in the Third Generation Partnership Project (3GPP) Technical Specification 3GPP TS 25.212, “Multiplexing and channel coding (FDD)” do not conform to this optimal condition. FIG. 2 provides an overview of the HSDPA RM procedure. The procedure is divided into two stages.
As seen therein, in the first stage 201, a rate ⅓ UMTS turbo codeword is rate-matched such that the output codeword can fit within a buffer size available at the receiver. If the original codeword length is smaller than the receiver buffer size, this stage can be transparent (i.e., output is identical to the input). This RM stage determines the effective mother code rate r0 in accordance with the following equation
                              r          0                =                              N            sys                                              N              sys                        +                          N                              p                ⁢                                                                  ⁢                1                                      +                          N                              p                ⁢                                                                  ⁢                2                                                                        (        1        )            
where Nsys, Np1, and Np2 are defined in Section 4.5.4 of 3GPP TS 25.212, “Multiplexing and channel coding (FDD)” (see also FIG. 2). In the second stage 202, the codeword is further rate-matched to the code rate specified by the current transmission format. The RM stage determines the initial code rate r1 in accordance with the following equation:
                              r          1                =                              N            sys                                N            data                                              (        2        )            
where Nsys and Ndata are defined in Section 4.5.4 of 3GPP TS 25.212, “Multiplexing and channel coding (FDD)” (see also FIG. 2).
For each of the QPSK and 16QAM modes in HSDPA, eight different RVs are defined in the 3GPP TS 25.212 by specifying parameters for the second-stage RM and the bit-to-symbol mapping. These definitions are repeated in the Tables 300 and 400 of FIGS. 3 and 4.
It will be seen that many of the HSDPA RVs are mixtures of all three types of HARQ protocols described above. It is hence necessary to refine and tailor the procedures to HSDPA operations. What is desired are new procedures and solutions to account for the specific and idiosyncratic properties of the HSDPA RVs. Solutions are required to overcome the following problems identified in the UMTS HSDPA Specification:
HSDPA RM agent repeats bits when there are still unused bits. Best RVs for IR operation need to be searched;
exact behaviors of the RM agent depend on the block lengths;
first stage RM effectively increases the mother code rate and, hence, decreases the gains of the IR protocols;
when bits can be repeated, proper prioritization between systematic and parity bits are needed; and
counter-measures against fast changing channel conditions are needed.
Thus, it would be advantageous to have a system and method that overcomes the cited disadvantages of the prior art. The present invention provides such a system and method.